Chlorinated hydrocarbons such as 1,1,1-trichloroethane and chlorfluoro hydrocarbons (CFC), such as C.sub.2 F.sub.3 Cl.sub.3 also known as FREON.RTM., have been used for many years as solvents in numerous applications. These materials have shown to be very useful in the preparation and delivery of activator and accelerator compositions used in the adhesive field. In recent years, the use of these compounds has been substantially lessened due to their deleterious effect on the environment. Regulations have been promulgated not only in the United States but in countries throughout the world to accelerate the phase-out of environmentally destructive solvents, and in particular those which are believed to be ozone-depleting substances. In addition to laws designed to prevent the use of these substances, product labelling requirements have also been promulgated to insure notice is given as to those compounds used as alternatives. Finding acceptable alternatives, however, has been an extremely difficult task. For example, in applications such as adhesive accelerator compositions, the solvent should possess high chemical stability, non-flammability, low toxicity, yet be sufficiently volatile to readily evaporate once applied to leave the active ingredient, e.g. an amine accelerator or activator, on the surface to which it is applied. In addition to these requirements, consideration must be given to cost factors which are critical to commercial development.
Attempts to find good solvent systems which are non-ozone depleting, non-flammable, yet sufficiently volatile, have not been entirely successful. Although many materials have possessed certain of these characteristics, many have been deficient in the majority of these properties. For example, materials such as heptane, acetone, methylethyl ketone, isopropanol, and methanol are excellent solvent systems, with excellent volatility, i.e. low flash point temperatures and high vapor pressures, but are extremely flammable. Other substances such as isoparaffins, and propylene glycol ether are less flammable, but are still combustible and evaporate slower.
In co-assigned and co-pending U.S. Ser. No. 08/322,937 filed concurrently herewith, one solution offered for the replacement of ozone-depleting CFC carriers in adhesive promoter compositions was a unique co-solvent composition comprising an azeotropic mixture of a perfluorocarbon compound and an alkylsiloxane. This co-solvent solution was non-ozone depleting and possessed the required characteristics to be useful as a carrier for adhesive promoter compounds, including activators, accelerators, initiators and the like for promoting cure or enhancing adhesion of cyanoacrylate adhesives.
It is conventional in the art of adhesive promoters, i.e. accelerators, activators, initiators and the like, that they be carried in volatile solvents when used as separate components in conjunction with adhesive compositions. In the case of cyanoacrylate adhesives, accelerators are generally applied to a substrate surface prior to application of the adhesive. These adhesive promoter compositions, sometimes also referred to as primer compositions, are usually comprised of amine compounds dissolved in a CFC solvent, or 1,1,1-trichloroethane, which as discussed above is no longer considered an environmentally safe material due to its ozone-depleting nature.
Numerous patents describe the use of volatile solvents as carriers for adhesive promoter compositions, such as activators, accelerators and the like. Such adhesive promoter compositions are used in conjunction with the cure of cyanoacrylate adhesives, olefinic adhesives, anaerobic adhesives and two-part acrylic systems. For example, U.S. Pat. No. 3,260,637 describes a method of bonding a cyanoacarylate adhesive using amines dissolved in a volatile carrier solvent preferably having a boiling point below about 100.degree. C. U.S. Pat. No. 4,822,426 discloses a primer solution consisting of an organometallic compound combined with a fluorine-containing compound, for use with difficult to bond to non-polar resins, such as polyolefins. The primer composition is applied with a volatile solvent.
U.S. Pat. No. 3,836,377 discloses an adhesive composition which consists of a monomeric ester of cyanoacetic acid and a polymerization accelerator which can be applied by means of a volatile solvent. U.S. Pat. No. 4,979,993 teaches the use of a primer composition consisting of a tertiary ammonium carboxylate compound carried by a low surface tension organic solvent. This reference also teaches that the solvent should readily evaporate to allow for the reaction of the primer with the adhesive.
In two-part adhesive compositions, where a curable component is present in one part and a curing agent is present in a second part, the curing agent and/or accelerator is generally applied with vehicles such as lower alcohols and in some cases water. For example, see U.S. Pat. No. 4,460,759, which teaches the removal of the solvent prior to bringing the surfaces together to initiate the bonding reaction.
U.S. Pat. No. 4,933,234 discloses primed polymeric substrates having at least one organic nucleophilic graft-polymerizable monomer or salt thereof grafter thereon. Various amine-bearing monomers are disclosed and the monomer compositions may be applied using solvents, crosslinking agents and surfacts.
Additionally, cyanoacrylate adhesives have incorporated a variety of additives directly into the adhesive composition per se to strengthen the bond. These additives have conventionally been directly incorporated into the adhesive composition and are not handled as a separate component, in contrast to typical accelerator and activator compositions. These additives, in the case of U.S. Pat. No. 4,139,639 included an adhesion promoter of the formula: ##STR1## wherein R' is hydrogen or an alkyl, aryl or cycloalkyl group having 1 to 10 carbon atoms. The formal name given to this class of compounds is 3,4,5-trihydroxybenzoic acid or esters thereof. In addition to the adhesion promoter, an inhibitor is also added to the cyanoacrylate adhesive composition to stabilize the mixture from prematurely curing.
U.S. Pat. No. 2,784,127 discloses cyanoacrylate adhesive compositions which are plasticized by the addition of alkyl ester in aliphatic dicarboxylic acid of the formula, R.sub.1 OOC--(CH.sub.2).sub.n --COOR.sub.1 wherein n is a whole integer in the range of 1-8 and R.sub.1 is an alkyl group from 1 to 8 carbon atoms. Also disclosed are alkyl phosphates in which each alkyl group contains from 1 to 8 carbon atoms, as well as triaromatic phosphates such as triphenylphosphate and tricresyl-phosphate. These plasticizers are dissolved in the monomeric cyanoacrylate adhesive to prevent failure of the adhesive bond after aging. Up to about 20% of the plasticizers are disclosed as being incorporated. The plasticizers allegedly function to add flexibility to the bond without adversely affecting the degree of adhesion. No disclosure is given to the addition of a promoter component into the adhesive composition per se.
As is evident from the discussion, the art is replete with examples of volatile solvents as carriers for adhesion promoter compositions. This is the case whether the adhesion promoter be an activator, accelerator, catalyst or initiator system. Solvents have not historically been welcome in the bonding process since they weaken the adhesive bond, interfere with the adhesive bonding process, prevent contact between the adhesive and the substrate and in some instances, attack the adhesive and/or the substrate.
The present inventor has discovered that using a fluid carrier other than a volatile solvent to dissolve the active component, carry it to the substrate and incorporate itself into the adhesive composition during bonding, alleviates these difficulties. Thus, rather than approach the environmental problem of ozone depletion using a non-ozone depleting volatile solvent, the present invention utilizes a substantially non-volatile fluid carrier which is miscible in the promoter, delivers the promoter component to the substrate surface and remains incorporated in the adhesive.
It is clear that there is a need for such a non-volatile, non-ozone depleting fluid carrier which is capable of sufficiently carrying the adhesive promoter component to the substrate surface, as well as being incorporated into the adhesive composition during the bonding process without interfering with polymerization.